1. Field of the Invention
The present invention relates to a process for the two-phase extraction of metal ions from phases containing solid metal oxides by means of a hydroxamic acid dissolved in an organic solvent. The present invention also relates to an agent for the extraction of metal ions from phases containing solid metal oxides. Finally, the present invention relates to the use of hydroxamic acids as agents for the extraction of metal ions from phases containing solid metal oxides.
2. Statement of Related Art
Processes for the selective extraction of metal ions from aqueous solutions with the aid of a hydroxamic acid dissolved in an organic solvent are known from the prior art.
According to DE-PS 22 10 106, using a hydroxamic acid of the general formula (A) ##STR1## in which the radicals R represent alkyl radicals, and in which the total number of carebon atoms in the molecule is greater than 10, transition metals are extracted from the partly radioactive, aqueous solutions from uranium-processing plants.
According to U.S. Pat. No. 3,464,784, the vanadium is extracted from aqueous solutions containing tetravalent vanadium with the aid of organosoluble hydroxamic acids of the general formula (B) ##STR2## in which R can represent alkyl, cycloalkyl or aryl radicals with 7 to 44 carbon atoms, preferably so-called "neo-alkyl radicals", which contain a quaternary carbon atom next to the carbonyl group.
"J. Chem. Research" (S) 1982, 90 ff, in addition, describes the solvent-extraction of transition metals with so-called versatohydroxamic acids of the above general formula (B), in which the radicals R are branched alkyl radicals containing 10 to 15 carbon atoms. The solvent-extraction with trialkylacethydroxamic acid of various metal isotopes from aqueous solutions from plants processing radioactive residues is described in "Reprints of the ISEC '86", Sep. 11-16, 1986, Munich, p. 355-362.
The processes described above, however, are solvent-extractions or liquid-liquid extractions, i.e. extraction processes in which the metal ions to be extracted are already in solution.
The separation of impurities from precious-metal electrolyte solutions by means of a two-phase liquid-liquid extraction is also known. The impurities arsenic, antimony, bismuth and iron are, for example, separated from aqueous, mineral-acid copper-electrolyte solutions with hydroxamic acids used as extracting agents. In this prior art, separation is effected by a liquid-liquid extraction, in which the impurities to be removed, which are in a dissolved form in the strongly acidic solution, are extracted in the organic phase.
The German Patent Application P 37 25 611.4 published on Feb. 9, 1989, for example, relates to a process for the simultaneous separation of arsenic, antimony, bismuth and iron from precious-metal electrolyte solutions by means of solvent extraction and the subsequent recovery of the named impurities, which is characterized in that aqueous, mineral-acid precious-metal electrolyte solutions are added to a poorly water-soluble organic solvent which contains one or more hydroxamic acids, the two phases are mixed together intensively, the impurities arsenic, antimony and bismuth are precipitated from the organic phase by sulfide-precipitation, the sulfides are separated off and the iron remaining in the organic phase is then re-extracted with a water-soluble complexing agent for iron into an aqueous phase and recovered.
The German Patent Application P 38 36 731.9 published on May 3, 1990 relates, further, to a process for the separation of impurities selected from arsenic, antimony, bismuth and/or iron from precious-metal electrolyte solutions by means of solvent-extraction and subsequent recovery of the named impurities, in which an aqueous, mineral-acid precious-metal electrolyte solution is added to a poorly water-soluble organic solvent containing one or more hydroxamic acids, the two phases are mixed together intensively, arsenic, antimony and bismuth are precipitated from the organic phase by sulfide-precipitation, the sulfides are separated off and the extracted iron is then re-extracted with a water-soluble complexing agent for iron into an aqueous phase and recovered, which is characterized in that prior to the sulfide precipitation, the organic phase is re-extracted with water over a sufficient contact time, the arsenic and/or antimony re-extracted into the water phase are optionally precipitated out by reduction in a manner known per se and processed.
Depending on the nature and composition of such precious-metal electrolyte solutions, the impurities, which are present in most cases as metal oxides before they are processed, can, however, also be undissolved in a solid form, i.e. suspended in the mineral-acid solution. In particular, solutions from refining electrolysis, especially those from copper-refining electrolysis, can contain impurities in the form of finely-dispersed solids. To carry out the solvent-extraction according to the prior art, however, clear electrolyte solutions without any suspended solids content are required. In the past, therefore, such solids have been removed from the electrolyte solutions, e.g. by filtration.
The elements arsenic, antimony and bismuth in the form of their metal oxides frequently also occur in the secondary streams of different processes, for example, in the smelting of the base metals copper, lead or iron. Such secondary streams can be: flue dusts, slags, metal sludges, SO.sub.2 -roaster gases, which are subjected to wet-cleaning, or certain wash-waters, which are finally treated in subsequent effluent-cleaning stages. Separation usually takes place via the corresponding oxides. For final storage in "special-refuse" dumps, conversion into poorly soluble compounds is required, in particular in the case of arsenic in the form of calcium arsenates or basic iron arsenates. For ecological reasons and due to the ever more restricted dumping space--which ultimately causes increases in the costs of special-refuse dumping--, for a number years more and more metallurgical products have been subjected, for example, to "de-arsenification".
In the future, more and more crude ores with higher contents of impurities, particularly arsenic, will be processed. As a result, the amounts of As.sub.2 O.sub.3 to be removed from the SO.sub.2 -roaster gases in smelting-metallurgical processing will increase sharply (literature: The Aqueous Chemistry of Arsenic in Relation to Hydrometallurgical Processes, R. G. Robins, CIM Meeting, Vancouver, August 1985, p. 1 to 26). The wet-cleaning of SO.sub.2 -roaster gases containing As.sub.2 O.sub.3 takes place with water and/or H.sub.2 SO.sub.4 acid solutions in washing towers. Arsenic is precipitated from such wash-waters, for example, with lime. Since, however, the wash liquors are strongly acidic and contain many contaminants, the chemical consumption is high and the product is unclean. Making safe, utilizing or storing such arsenic-containing products constitutes a technical-economic problem. This also applies to the elements bismuth and antimony.
The problems are not limited only to the separation of these elements as "impurities". They are undesirable as impurities because they downgrade the quality of the "pure" metals produced mainly by refining and are also questionable from the ecological viewpoint. At the same time, the aforementioned secondary streams in the smelting processes of base metals are also important raw-material sources for obtaining these metals, which are put to use in various fields (electronics, optics, catalyst technology, as alloy constituents). As couple products, those oxides in particular are important, which form during the smelting of crude ores, for example: As.sub.2 O.sub.3 from roaster gases after wet-cleaning in copper, lead and iron smelting; Sb.sub.2 O.sub.3 from roaster gases after wet-cleaning, Sb-reclamation in lead works is particularly important, as is the reclaiming of heavy metals in subsequent pyro-metallurgical, wet and electrochemical refining processes; Bi.sub.2 O.sub.3 particularly from lead and copper smelting.
Processes are therefore sought which on the one hand separate off impurities such as, for example, As, Sb and Bi and thus minimize environmental impact and improve the product quality of the base metals produced, but which on the other hand also return these impurities to the economic cycle as useful materials (recycling). The problems described above are not, however, limited exclusively to the named elements As, Sb and Bi. They rather refer in addition to a number of other metals which occur in numerous processes in the oxide form as side constituents. Here too there is an urgent desire to be able to reclaim these metals and likewise to recycle them for further use.
To recover or reclaim any of these metals by means of a solvent-extraction process, according to the prior art it was hitherto always necessary first to bring the existing metal oxides into solution, which required the use of large volumes of treatment agents, especially mineral acids. These treatment agents then also had to be re-processed or disposed of.